Radar sensors are used in motor vehicles, for example, for measuring the distances and relative velocities of preceding vehicles, so that, for example, a collision warning and/or an automatic distance regulation is made possible. In the case of these applications, the radar sensor is installed in the front area of the vehicle body. The radome has the purpose of protecting the sensitive electronic components of the radar sensor against mechanical impact and weather conditions. In several known radar sensors, the radome is formed as a radar lens, which is used to simultaneously achieve collimation and beam forming of the radar beam. In the case of other known radar sensors the beam forming is solely carried out by the geometry and activation of the antenna elements, for example, according to the principle of a phased array antenna. In this case, the radome may be formed simply by a plane wall made of plastic.
Since the bumpers of vehicles today are usually made of plastic and are consequently transparent to radar radiation, it is frequently desired to install the radar sensor protected and concealed behind the vehicle's bumper. However, a problem exists that while the bumper made of plastic is largely transparent to the radar radiation, it does have a certain reflective capacity. Since, on the other hand, the incoming radar waves are also reflected on the mass structures of the printed circuit board after having passed through the bumper, multiple reflections may occur between the printed circuit board and the vehicle's bumper. These multiple reflections represent an undesirable interfering signal when the radar signal is evaluated.
Such multiple reflections are particularly interfering in the case of angle-resolving radar sensors, in which multiple antenna elements are situated side by side across the width of the vehicle, so that it is possible to measure or at least estimate the azimuth angle of the located object by evaluating the amplitude and phase relations between the radar echoes received by the different antenna elements. The multiple reflections may falsify the amplitudes and phases so severely that the accuracy of the angle measurement is considerably adversely affected.
A possible countermeasure is to tilt the bumper in such a way that its reflective surface is no longer parallel to the plane of the printed circuit board and consequently the radar waves no longer strike the antenna structures after one or repeated reflections. However, such a tilt of the bumper limits the constructional flexibility in designing the bumper and in installing the radar sensor.